It says Pornhub - a site based in Luxembourg - received 112,000 visits from British boys aged 12 to 17 during one month last year.

The new system would force these boys to provide credit card details or official records to prove they are 18 or over.

Websites failing to comply could be blocked in the same way as access is denied to sites hosting child pornography.

Officials in Downing Street and the Department for Culture, Media and Sport are studying how sites based overseas could be forced to adopt a similar system.

The rules would make porn websites follow the same rules as newsagents selling adult magazines - or booze, cigarettes and lottery tickets.

A spokesman for the Department for Culture, Media and Sport said: "We are legislating to put R18 material that can only be sold in licensed sex shops behind access controls on video-on-demand services.

"The same protections should apply online as offline and we are clarifying the law to make sure that is the case - we want the legislation to be in place by the end of the year.